Anatoli Shlapakovski

Investigations of a Double-Gap Vircator at Submicrosecond Pulse Durations

The results of investigations of a double-gap vircator driven by a 20 Ω and 500-ns generator operating in the output voltage range 400-600 kV are presented. The vircator generated microwave pulses with a peak power of up to 200 MW at ~5% efficiency and the frequency varied from 2.0 to 2.3 GHz depending on the cavity geometry. The limitations on the microwave pulse duration not related to the cathode plasma expansion are addressed. On the one hand, the microwave generation is terminated because of the plasma formation at the foil separating the cavity sections, so that the virtual cathode (VC) electron space charge is neutralized by the plasma ion flux. On the other hand, if the electron beam energy deposition into the foil is reduced, a substantial delay in the start time of the microwave generation appears, which has been studied in detail. With these limiting factors, the microwave pulse full duration varied from 100 to 350 ns; the maximal full width at half maximum duration achieved in the experiments was ~180 ns. Measurements of the current transmitted through the vircator cavity indicated the existence of a VC in spite of the absence of microwave generation during the delay. The experimental dependence of the microwave generation starting current on the diode voltage is presented, and possible mechanisms behind the generation delay are discussed. Simplified numerical simulations emphasize the role of the portion of electrons that are reflected from the VC, the number of which must be sufficient for the microwave generation to occur.

Effects of Different Cathode Materials on Submicrosecond Double-Gap Vircator Operation

The operation of a double-gap S-band vircator has been investigated at submicrosecond duration of a high-current electron beam generated in a planar diode. The experiments were performed at accelerating voltages of les550 kV and diode currents of up to 17 kA using a radio-frequency cavity with a wide coupling window between its two sections. Three types of cathodes have been studied, namely, metal-dielectric, carbon fiber, and velvet cathodes. The main features of the operation of the vircator using each cathode are analyzed. The microwave pulse duration with the metal-dielectric and carbon fiber cathodes reached ~250 ns at the peak power level of ~100 MW; with the velvet cathode, a duration of ~400 ns was achieved. It has been found that, in addition to the common limitations of the microwave pulse duration related to the dynamics of the diode impedance governed by the cathode plasma expansion, there is another factor, namely, the anode-cathode gap, which determines the delay at the beginning of the microwave generation. The latter effect is explained by the role of electrons oscillating between the virtual and real cathodes in the generation process. The issue of radiated microwave frequency behavior is discussed as well.

High power microwave source for a plasma wakefield experiment

The results of the generation of a high-power microwave (∼550 MW, 0.5 ns, ∼9.6 GHz) beam and feasibility of wakefield-excitation with this beam in under-dense plasma are presented. The microwave beam is generated by a backward wave oscillator (BWO) operating in the superradiance regime. The BWO is driven by a high-current electron beam (∼250 keV, ∼1.5 kA, ∼5 ns) propagating through a slow-wave structure in a guiding magnetic field of 2.5 T. The microwave beam is focused at the desired location by a dielectric lens. Experimentally obtained parameters of the microwave beam at its waist are used for numerical simulations, the results of which demonstrate the formation of a bubble in the plasma that has almost 100% electron density modulation and longitudinal and transverse electric fields of several kV/cm.

Plasma density temporal evolution in a high-power microwave pulse compressor switch

Time-resolved optical-emission spectroscopy measurements are used to evaluate plasma density in an interference switch during the extraction of a nanosecond output pulse from a high-power microwave compressor. The compressor represents a resonant cavity connected to an H-plane waveguide tee with a shorted side arm filled with helium at a pressure of ; the plasma discharge in the tee side arm is triggered by a Surelite laser. A nanosecond-scale dynamics of the plasma density is obtained by analyzing the shape of the helium spectral lines. The analysis of the experimental data evidences a correlation between the rise time of the plasma density and the peak power of the microwave output pulse. Numerical simulations of the microwave energy release from the cavity with the appearance of the plasma yield results in good agreement with the measured output pulse peak power and waveform.

Fast-Framing Optical Imaging of Plasma Formation in Resonant Microwave Pulse Compressor

Plasma evolution in the interference switch of an S-band pulse compressor operating in the frequency of 2.766 GHz, with input pulses of 200-450-kW power and duration of 2.4 μs, was studied experimentally and in numerical simulations. The system was filled with dry air at 2 × 105-3 × 105-Pa pressure. The plasma discharge that switches the phases of the compressor operation from energy storage to release was initiated by a Surelite laser. The evolution of the light emission from the plasma was studied using fast-framing optical imaging with a 4QuikE camera. From the obtained typical size of the plasma and its velocity of expansion along the electric field, the density of the plasma was estimated, and the influence of its evolution on the power and waveform of microwave output pulses observed in the experiments was determined in simulations.

Self-consistent evolution of plasma discharge and electromagnetic fields in a microwave pulse compressor

Nanosecond-scale evolution of plasma and RF electromagnetic fields during the release of energy from a microwave pulse compressor with a plasma interference switch was investigated numerically using the code MAGIC. The plasma was simulated in the scope of the gas conductivity model in MAGIC. The compressor embodied an S-band cavity and H-plane waveguide tee with a shorted side arm filled with pressurized gas. In a simplified approach, the gas discharge was initiated by setting an external ionization rate in a layer crossing the side arm waveguide in the location of the electric field antinode. It was found that with increasing ionization rate, the microwave energy absorbed by the plasma in the first few nanoseconds increases, but the absorption for the whole duration of energy release, on the contrary, decreases. In a hybrid approach modeling laser ignition of the discharge, seed electrons were set around the electric field antinode. In this case, the plasma extends along the field forming a filament and ...

Revisiting Power Flow and Pulse Shortening in a Relativistic Magnetron

The behavior of a six-vane relativistic magnetron with a single radial output slot has been studied in detail by 3-D particle in cell simulations. It is found that a delicate power imbalance caused by the pulsed-power generator and magnetron impedance mismatch is responsible for the shortening of the radiated power pulse. Pulse shortening can be avoided completely, and the radiated power can be considerably increased by decreasing the emitted current when it is too large to be sustainable. When the magnetron current is too high, the voltage decreases, which causes the radiated power to drop and shortens the pulse. This relatively simple electrical power balance is the result of intricate dynamics involving the electron flow within the magnetron, out of the interaction cavity through the axial flow and the electromagnetic modes supported by the structure.

Resonant microwave pulse compressor operating in two frequencies

A resonant microwave pulse compressor with a hybrid (Magic) waveguide tee as an interference switch was studied in numerical simulations and experimentally. In this compressor, the necessary condition for energy storage in the compressor cavity is frequency-independent, so that its operation in different cavity eigenmodes without mechanical tuning is possible. An S-band compressor operating in two different frequencies (neighboring modes) was investigated. Two characteristic geometries corresponding to different regimes of the microwave energy accumulation and release were tested using input pulses of 200–400 kW power, 2.4 μs duration, and variable frequency, 2.8 to 2.9 GHz. The geometries are characterized by an RF electric field in the interference switch that is higher or lower than the field in the cavity. The plasma discharge that switches the phases of compressor operation from energy storage to release was initiated by small metallic cones placed in the appropriate location. For both geometries, th...

Observation of Plasma at the Quartz Rod Inside Annular Electron Beam Produced From a Knife-Edge Cathode in a Magnetic Field

Time-resolved light emission imaging was used to observe the plasma formation at the surface of a dielectric rod serving as a slow-wave supporting structure in an antenna-amplifier Cherenkov maser configuration. Experiments were performed using a quartz rod inserted into the hollow knife-edge cathode of a magnetically insulated foilless diode generating an annular electron beam of 0.9-1.7-kA current. The accelerating voltage pulse was delivered to the diode by a linear induction accelerator; the voltage amplitude ranged from 260 to 380 kV at the full pulse duration of ~ 200 ns. It was shown that the plasma at the rod surface is produced in the vicinity of the cathode edge plane corresponding to the location of the strong tangential electric field component. This plasma appears later than the explosive emission plasma at the cathode edge, and the intensity of light from this plasma increases, while the voltage rises. At voltages below 300 kV, the measured side-view light intensity drastically decreases, which indicates a significant decrease in the plasma density. It has been found that the surface plasma does not propagate along the rod; yet, the presence of plasma at a distance of 5-10 cm from the cathode was registered sporadically at high voltages. The electron emission from the surface plasma was observed as well.

Wakefield in a waveguide

The feasibility of an experiment which is being set up in our plasma laboratory to study the effect of a wakefield formed by an ultra-short (≤10−9 s) high-power (∼1 GW) microwave (10 GHz) pulse propagating in a cylindrical waveguide filled with an under-dense [(2–5) × 1010 cm−3] plasma is modeled theoretically and simulated by a particle in cell code. It is shown that the radial ponderomotive force plays a circular key role in the wakefield formation by the TM mode waveguide. The model and the simulations show that powerful microwave pulses produce a wakefield at lower plasma density and electric field gradients but larger space and time scales compared to the laser produced wakefield in plasmas, thus providing a more accessible platform for the experimental study.